Fair Value of Financial Instruments (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	$ 441,186,000	$ 363,542,000
Equity method investment, ownership percentage	25.00%	25.00%
Equity method investment	$ 2,800,000	$ 3,600,000
Money market funds
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	32,815,000	10,065,000
U.S. Government bonds and notes
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	169,880,000	39,086,000
Corporate notes, bonds and commercial paper
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	238,491,000	314,391,000
Recurring basis
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	441,186,000	363,542,000
Recurring basis | Money market funds
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	32,815,000	10,065,000
Recurring basis | U.S. Government bonds and notes
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	169,880,000	39,086,000
Recurring basis | Corporate notes, bonds and commercial paper
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	238,491,000	314,391,000
Recurring basis | Quoted market prices in active markets (Level 1)
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	32,815,000	10,065,000
Recurring basis | Quoted market prices in active markets (Level 1) | Money market funds
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	32,815,000	10,065,000
Recurring basis | Quoted market prices in active markets (Level 1) | U.S. Government bonds and notes
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	0	0
Recurring basis | Quoted market prices in active markets (Level 1) | Corporate notes, bonds and commercial paper
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	0	0
Recurring basis | Significant other observable inputs (Level 2)
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	408,371,000	353,477,000
Recurring basis | Significant other observable inputs (Level 2) | Money market funds
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	0	0
Recurring basis | Significant other observable inputs (Level 2) | U.S. Government bonds and notes
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	169,880,000	39,086,000
Recurring basis | Significant other observable inputs (Level 2) | Corporate notes, bonds and commercial paper
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	238,491,000	314,391,000
Recurring basis | Significant unobservable inputs (Level 3)
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	0	0
Recurring basis | Significant unobservable inputs (Level 3) | Money market funds
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	0	0
Recurring basis | Significant unobservable inputs (Level 3) | U.S. Government bonds and notes
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	0	0
Recurring basis | Significant unobservable inputs (Level 3) | Corporate notes, bonds and commercial paper
Financial assets subject to fair value measurements and the necessary disclosures
Total available-for-sale securities	$ 0	$ 0